The present invention relates to the downhole separation of gas and liquid from an oil-gas production stream and in one aspect relates to separating a portion of the gas from a gas-oil production stream downhole in a wellbore while recovering increased amounts of valuable condensate to the surface along with the liquids in the remaining production stream.
It is well known that in many oil wells, extremely large volumes of gas are produced along with the crude oil and other formation fluids, e.g. water. In such wells, it is not unusual to experience gas-to-oil ratios (GOR) as high as 20,000 standard cubic feet per barrel (scf/bbl.) or greater. Since the gas and oil are commingled and are produced to the surface as a single production stream, large and expensive equipment is required at the surface to separate this gas from the liquids in the production stream before either can be further processed and/or sent on to market. In fact, the amount of gas in some production streams may be so large that the available surface equipment can not handle the load and production may have to be cut back or the well shut-in at significant expense.
To reduce the size of the equipment and the related costs normally involved in separating such large volumes of gas from a production stream, various methods and systems have now been proposed wherein some of the separating/handling steps normally required at the surface are carried out downhole before the production stream is brought to the surface. These methods all basically involve separating at least a portion of the gas from the production stream downhole and then handling the separated gas and the remainder of the production stream (i.e. liquids) separately.
For example, one such method involves the positioning an xe2x80x9caugerxe2x80x9d separator downhole within a production wellbore which separates a portion of the gas from the production stream as the stream flows upward through the auger separator; see U.S. Pat. No. 5,431,228, issued Jul. 11, 1998. Both the remainder of the production stream and the separated gas are flowed to the surface through separate flowpaths where each is individually handled.
This type of auger separator now form an integral part of several different downhole gas-separation systems which are sometimes called xe2x80x9cSubsurface Processing And Reinjection Compressor systemsxe2x80x9d or xe2x80x9cSPARCxe2x80x9d. For example, in one such system, an auger separator is used to separate at least a portion of the gas from the production stream which, in turn, is then recompressed downhole and injected into a subterranean formation without ever producing the separated gas to the surface; see U.S. Pat. No. 5,794,697, issued Aug. 18, 1998. Another SPARC system utilizing an auger separator is disclosed in U.S. Pat. No. 6,189,614, issued Feb. 20, 2001 when a SPARC separates and compresses a portion of the gas in the production stream basically in the same manner as described above, but instead of re-injecting the compressed gas, both the compressed gas and the remainder of the production stream are produced to the surface through separate flowpaths.
While such downhole auger separators can separate relatively large volumes of gas from a production stream, unfortunately, they may also separate with the gas, a substantial amount of any condensate (i.e. volatile liquids) which may be present in the production stream. As will be understood in this art, it is much more desirable to recover this valuable condensate as part of the liquid stream rather than xe2x80x9closexe2x80x9d it into the separated gas stream. As used herein, xe2x80x9cliquid streamxe2x80x9d is intended to mean the production stream which remains after the separated gas has been removed.
Recently, a downhole SPARC system has been proposed which is adapted to increase the amount of condensate which can be recovered with the liquid stream; see commonly-assigned U.S. patent application Ser. No. 09/351,483, filed Jul. 13, 1999 and now U.S. Pat. No. 6,260,619 B1, issued Jul. 17, 2001. In this system, the amount of recovered condensate is increased by flowing both the separated gas stream and the liquid streams through a restrictive passage after they pass through the auger separator and before the gas is flowed into a downhole compressor.
In downhole gas separation systems such as those discussed above, the auger separator, itself, is basically designed to separate as much gas from the production stream as is practical in a given situation without any real concern being given to the amount of condensate which is also separated with the gas. Again, it will be understood that it is much more desirable from an economic standpoint to retain as much of the condensate as possible in the liquid stream as the production stream passes through the downhole auger separator.
The present invention provides a method and system for producing a mixed gas-oil stream to the surface from a production zone through a wellbore wherein at least a portion of said gas is separated from said mixed gas-oil stream downhole while the amount of condensate in the remaining gas-oil stream is increased over what it normally would be. Basically, at least a portion of the gas is separated from the mixed gas-oil stream downhole as the mixed stream flows upward through a converging restrictive passage within a separator which is positioned in the wellbore.
As the stream flows through the restrictive passage in the separator, the velocity of the gas-oil stream is increased which, in turn, reduces the temperature and pressure of the gas-oil stream to thereby increase the amount of condensate which will remain in the liquid or remaining mixed gas-oil stream. The separator is mounted at the lower end of a tubing string wherein the liquid stream is produced to the surface through the tubing. The separated gas stream is then flowed through a diverging passageway in the separator to thereby recover at least a portion of the pressure lost when the gas was separated within the restrictive passageway within said separator and is either produced to the surface through the well annulus or is fed to the compressor of a SPARC unit for injection in a downhole zone.
More specifically, the present invention provides a system for producing a mixed gas-oil stream from a production zone to the surface through a wellbore wherein the system comprises a string of tubing which is positioned within the wellbore wherein an annulus is formed between said tubing and said wellbore. The tubing string carries an auger separator at its lower end which is adapted to separate at least a portion of the gas from the remainder of the gas-oil stream downhole as the gas-oil stream flows upward through the wellbore.
The auger separator has a housing which is mounted in the tubing whereby an annular passage is formed between the housing and the tubing. An auger is mounted in the housing and has an upstream section and a downstream section. The upstream section has a tapered core (e.g. conical-shaped) whose diameter increases upwardly from the bottom towards the top thereof whereby the passageway formed between the core of the upstream section and the wellbore converge upwardly throughout the upstream section. An auger flight is secured to and along the core of the upstream section and extends outward substantially to the wall of the tubing.
The downstream section of said separator is adapted to be secured in the lower end of the tubing string and has a tapered core (e.g. conical-shaped) whose diameter decreases upwardly from the bottom towards the top thereof whereby the passageway formed between the core of the downstream section and the tubing diverge upwardly throughout the upstream section
In operation, the mixed gas-oil stream flows upward through the converging passageway in the upstream section of the downhole separator where the auger causes the mixed stream to rotate with the heavier liquid components moving outward while gas remains near the core of the separator. Flow through the converging or restrictive passageway in the upstream section increases the velocity, hence lowers the temperature and pressure, of the stream. By reducing the temperature and pressure of the stream, more of the desired condensate in the mixed stream remains in the liquid stream which is forced outwardly from the gas stream by the centrifugal forces imparted by the auger separator. The liquid stream is then produced to the surface through the tubing string. Where the downhole system includes a SPARC unit, the liquid stream flows through the turbine section thereof to thereby drive the turbine which, in turn, rotates the compressor of the unit.
The separated gas flows upward through the diverging passageway formed between the tapered core of the downstream section and the tubing string where the gas regains at least a portion of the pressure lost during flow through the converging passageway in the upstream section of the separator. The separated gas is then either produced to the surface through the well annulus or is fed into the compressor of the SPARC unit, if present, where it is compressed for injection into a downhole zone.